This invention relates to a method for forming pattern.
Recently, with the promotion of higher density integration of semiconductor devices, the light source wavelength of an exposure device used for fine process, especially photolithography is more and more shortened. At present i-line (365 nm) has entered its practical use and KrF excimer laser (248.4 nm) is already investigated. However, resist pattern materials, especially resist materials suitable for KrF excimer laser, and deep ultraviolet wavelength ranges have not been developed sufficiently. For example, even when MP2400 (Shipley Co.) which is said to have high sensitivity and transmittance for KrF excimer laser light is used, because of the surface absorption of a novolac resin which is a base polymer and poor optical reactivity of a sensitizer, naphthoquinone diazide compound, pattern shape after pattern formation is too low in quality to be used.
Also as a pattern forming material for deep UV, there is reported a resist which contains 2-diazo-1,3-dione compound which has a high transmittance for deep UV light of around 248 nm. However, compared with the transmittance of 70% of the base polymer of the resist, the transmittance is only approx. 40%, and enough photo-bleach is not obtained. Also as a result of pattern forming experiments, it is found that the pattern has an angle of approx. 70 degrees which value is insufficient compared with a pattern shape which becomes a satisfying etching mask with a vertical shape.
Also it has become clear that the sensitivity of this pattern forming material is as low as from 140 to 150 mJ/cm.sup.2. That is, the high transmittant pattern forming material containing a 2-diazo-1,3-dione compound has low sensitivity, and is difficult for a practical use when there is used KrF excimer laser light of which energy efficiency is poor.
In recent years, as a means to decrease exposure energy quantity, a material comprising poly(tertial-butoxy carbonate (t-BOC)) styrene and an onium salt was proposed. This is a chemical amplification pattern forming material which generates an acid by exposure to light, said acid acting as a catalyst. Various reports have been made recently [e.g. Polym. Eng. Sci. vol. 23, page 1012 (1983)]. A pattern forming method using such a chemical amplification pattern forming material is explained referring to FIGS. 1(a) to 1(d). The pattern forming material 12 is spin coated on a semiconductor substrate 1, and it is soft baked for 90 seconds on a hot plate of 90.degree. C. to obtain a 1.0 .mu.m thick pattern forming material film (FIG. 1(a)). In most cases, an insulation film, an electroconductive film and an oxide film are formed on the substrate 1. Next, an acid is generated from a photoacid generator in the material 12 as shown in the following chemical change by exposure to KrF excimer laser (248.4 nm) 4 through a mask 5 (FIG. 1(b)). ##STR1## Then the chemical change mentioned below is caused by the heat treatment (post exposure bake) of the said material film on a hot plate for 90 seconds at 130.degree. C., and the resin becomes alkali-soluble (FIG. 1(c)). ##STR2## Finally, positive type pattern 12a and 12c are obtained by dissolution and removal of exposed part 12 of the pattern forming material 12 by using an alkaline developer (MF-319, mfd. by Shipley Co.) (FIG. 1(d)).
But, it was proved impossible to apply this method for forming fine pattern 12c on a substrate as shown in FIG. 1(d), if the pattern size is 1.0 .mu.m or less, especially 0.5 .mu.m or less. The broken line in FIG. 1(d) shows that the pattern which is to be retained is not retained. The reason why the super fine patterns are not formed is proved to be the low adhesiveness between the pattern forming material and the substrate according to the present inventors' investigation. Although this is not a problem in a device production of several .mu.m level as in said prior art process, it is a fatal problem in the process for forming fine patterns of 1 .mu.m or less, especially super fine patterns of 0.5 .mu.m or less in high density. Consequently, it is impossible to produce a device of submicron rule. Thus, the reason why the super fine patterns are not formed turned out to be the low adhesiveness between the pattern forming material and the substrate. Since poly(t-BOC)styrene resin conventionally used for a pattern forming material does not contain the hydrophilic radical in its molecule, when it is made into a thin film, it becomes hydrophobic. As for the substrate, hydrophobic treatment with hexamethyldisilazane (HMDS) is applied to make the substrate surface uniform before forming the pattern forming material film. Thus the substrate surface becomes hydrophobic. It became clear by the present inventors' investigations that since the hydrophobic substrate and the hydrophobic pattern forming material have poor adhesiveness each other, when the exposed part is dissolved by a developer and removed, an unexposed part which is to be not dissolved is also removed due to pood adhesiveness, resulting in failing to form patterns on the substrate. This phenomenon becomes especially remarkable in fine patterns of 1 .mu.m or less. Therefore, it is extremely important to prevent this in the production of super fine semiconductor integrated circuits which forms super fine patterns of 1 .mu.m or less, especially 0.5 .mu.m or less, with high productivity.
It has been proved that the pattern size of these chemical amplification type resists change by heating the substrate (the said PEB (post exposure bake)) to disperse the generated acid after exposure as shown in FIG. 2. For instance, the size change caused by PEB performed one hour after exposure is over 20%, and when this is applied to the production of semiconductor device of super fine rule, pattern sizes between wafers on chips becomes uneven affecting greatly on productivity, reliability and characteristics of the semiconductor elements, accordingly it is impossible to form a device as designed. The present inventors found that the reason of pattern size change is that the functional group in the resin shows alkali soluble reaction during the exposure.
Conventional poly(t-BOC)styrene resin shows the below reaction which is the same one as under acid ambience by deep ultraviolet. ##STR3## As seen above, the resin becomes to show the alkali-soluble property in this exposure process before dispersing the generated acid by exposure through the PEB process. Therefore, the pattern size changes depending on the period between the exposure and the PEB. Poly(t-BOC)-styrene has two points where the linkage is weak (shown by .uparw.) in the functional group. ##STR4## The linkages are easily cut off not only by acid ambience but also by exposure to light. That is, because the t-BOC moiety has many points where the linkage is weak, separation by exposure easily occurs. In short, this is considered to cause the size change. Consequently it is extremely important to prevent this in the production of a semiconductor integrated circuit with super fine patterns of which size allowance is small.